Previous work from this laboratory has indicated that the purine nucleoside inosine has important effects in the normal and the ischemic heart. Thus, inosine (1) increased contractile force of nonischemic and ischemic mycardium, (2) increases coronary and coronary collateral blood flow following acute coronary occlusion and (3) substantially reduces infarct size after coronary ligation. These studies have also shown that the increase in contractile force of ischemic heart is not associated with a preservation of high energy phosphate stores and that following coronary occlusion, the endogenous inosine level in ischemic muscle becomes sufficiently high to influence the contractile state of the muscle. Presently, studies are proposed to study the effects of hypoxanthine, a related purine, on the heart and coronary circulation. Also, studies are proposed to determine whether inosine has a direct vasodilating action on the coronary circulation or whether the increased coronary flow seen with inosine is entirely secondary to an increase in cardiac work.